VOR navigation: radials, the CDI and which way to turn

What is a VOR and how does it work?

VOR stands for VHF omnidirectional range. Each station transmits on a very high frequency (VHF) channel between 108.00 and 117.95 MHz, and the receiver in the aircraft works out the magnetic bearing from the station to itself. There are 360 of these bearings, one for every degree, and each one is called a radial. The 090 radial runs east from the beacon, the 180 radial south, and so on. A radial is a line on the ground: you are on it or you are not, regardless of where the nose is pointing.

Because it is VHF, the signal travels in straight lines. Range is limited by line of sight, so the higher you fly the further out you can receive it, and high ground between you and the station can block it. Each VOR has a published coverage, and outside it the indications are not to be trusted even if the needle looks steady.

Every station sends a two- or three-letter identifier in Morse code. Listen to it and check it against the chart before you use the needle for anything. A station under maintenance may send a test code or no ident at all, and then it must not be used. Many VORs have distance measuring equipment (DME) alongside, which adds the range to the picture; the DME arc guide covers that.

In the UK the network is shrinking as routes move to area navigation (RNAV), and a number of en-route VORs have been or will be withdrawn. The skill still matters: VORs remain in the exams, in instrument approaches at many airfields, and as the backup when satellite navigation is unavailable.

The OBS, the CDI and the TO/FROM flag

The basic cockpit instrument has three parts. The omni bearing selector (OBS) is the knob that rotates a course card; whatever sits under the index is the course you have selected. The course deviation indicator (CDI) is the vertical needle. The TO/FROM flag is a small window showing TO, FROM, or OFF (sometimes a red flag) when there is no usable signal.

On a traditional five-dot indicator, the needle goes to full scale at about 10° off the selected course, so each dot is about 2°. Degrees become distance as you move away from the station: by the 1-in-60 rule, 2° at 60 nautical miles (NM) is 2 NM, but 2° at 6 NM is only a few hundred metres. That is why the needle feels sluggish far out and lively close in. Many horizontal situation indicators (HSIs) and glass cockpit displays show two dots a side for the same 10°, so there one dot is about 5°. Check which you have before you start counting dots.

The TO/FROM flag answers one question: if you flew the course you have selected, would it take you towards the station or away from it? Picture a line through the station at right angles to the selected course. On one side the flag reads TO, on the other FROM, and close to that line it shows OFF. The aircraft’s heading plays no part. Turn the aeroplane round and neither the needle nor the flag moves.

Which way do I turn? Towards the needle, with the right course set

The needle shows where the selected course is relative to you. Needle right: the course is to your right. If the course you have set is roughly the direction you are flying, turn towards the needle and it will come back to the centre. That is the whole rule, and it works as long as the set-up is right.

The trap is reverse sensing. Suppose you are tracking outbound on the 360 radial with 360 selected and FROM showing, and you turn round to head back to the station without touching the OBS. The needle is still correct for the course you selected, but you are now flying its reciprocal, so left and right have swapped. Following the needle takes you further off with every mile. The fix is not to learn to fly it backwards: select 180, the course you are actually flying. The flag changes to TO and the needle now reads the right way.

Try it: Open the VOR/ADF trainer and pick the “The reverse sense” demonstration to watch the needle stay put while the aircraft turns round

Which radial am I on?

Orientation is the first question on any VOR exercise and a standard exam item. You do not need to fly any particular heading to answer it, because the instrument does not know your heading.

1. Tune the frequency and identify the Morse ident against the chart.
2. Turn the OBS until the needle centres.
3. If the flag reads FROM, the number under the index is the radial you are on. If it reads TO, the number is the course to the station, and the radial is its reciprocal.
4. For a fix, repeat with a second station and plot both radials: you are where the lines cross.

The needle centres twice as the OBS goes round, 180° apart, once with FROM and once with TO. Both describe the same line on the ground; the FROM reading gives you the radial directly. The trainer’s “Finding the radial you are on” demonstration walks through exactly this.

How to intercept a VOR radial

To join a radial you fly an intercept heading that crosses it at an angle, then turn onto the course as the needle centres. The steps are the same inbound or outbound.

1. Identify the station and find which radial you are on now.
2. Set the course you want to fly on the OBS: the radial itself if you are going outbound (FROM), or its reciprocal if you are going inbound to the station (TO).
3. Note which side the needle is on. Turn towards that side onto an intercept heading: the course plus or minus your intercept angle.
4. Hold that heading. The needle comes off the stop and moves towards the centre; start the turn onto the course just before it centres, earlier the closer you are to the station, so that you roll out with it centred.
5. Set a heading that allows for the wind, then track and correct as below.

A 30° intercept is a sound default: quick enough to get you there, gentle enough that you can roll out without flying through. Further from the station, or when the radial is a long way off, 45° or more saves time. Close in, the needle moves fast, so use a smaller angle and start the turn early.

The VOR/ADF trainer shows the needles from any position you drag the aircraft to, but it does not have a dedicated intercept exercise. For a flown intercept, the instrument rating course lesson “Intercept and track a radial, TO and FROM” joins a radial inbound with a 30° cut, passes over the station and leaves outbound on another radial.

Tracking a radial in wind: bracketing

Once you are established, the wind will push you off the radial unless you point the nose a little into it. If you have a forecast wind you can estimate the correction first; the wind correction angle guide shows how. If not, find it by bracketing. The second half of the animation above does exactly this in a wind from the north.

• Fly the course heading with no correction and watch. The needle moves towards the upwind side, because the wind has blown you off the other way.
• Turn towards the needle, typically 20–30°, and hold it until the needle is back in the centre.
• Now turn back to about half that correction, into wind, and watch again. If the needle drifts the same way, add a few degrees. If it drifts the other way, take a few off.
• Each correction smaller than the last, until you find the heading that keeps the needle still. That heading, not the course, is what you fly.

Try it: Fly the course lesson “Find your radial, then track it inbound in a crosswind”, which finds the drift by bracketing in a 25 kt crosswind

Station passage, the cone of confusion, and the ADF

Directly above the beacon the receiver cannot resolve a bearing. As you approach, the needle becomes very sensitive, then swings, and the flag flickers or shows OFF before settling on FROM. This region is the cone of confusion, and it is wider the higher you are. Do not chase the needle through it: hold the heading you were flying and wait for the flag to change to FROM. That first steady FROM is your station passage; the OFF flag only tells you that you have reached the cone, not that you are past the beacon. Then set the outbound course, if it differs, and intercept it.

An HSI does the same job as the CDI but draws the course needle on a compass card that turns with the heading, so the picture is laid out the way the aircraft is pointing and the deviation bar always sits on the side where the course really is. That makes it much easier to read. The glass cockpit version on a primary flight display (PFD) works the same way.

The trainer also includes an automatic direction finder (ADF), a different instrument working from a non-directional beacon (NDB). Its needle points at the beacon relative to the nose, and heading plus relative bearing gives the magnetic bearing to the beacon, the QDM. With a card that turns with the heading, as on a radio magnetic indicator (RMI), the needle reads that bearing directly. En-route NDBs are being withdrawn in the UK too, but the principle still appears in the exams.

Put the three side by side and move the aeroplane yourself: turn it round and see which needles care.